Field of the Invention
This invention relates to an electrically tunable solid-state microwave oscillator and more particularly to an oscillator having a two-inductor tunable series circuit for a wide range of frequency tunability.
Description of the Prior Art
A continuing need exists for wide-band electrically tunable, solid-state microwave oscillators wherein an active element generates a signal of a particular frequency in the microwave frequency bands dependent upon electrical circuit parameters which form a resonant circuit. Tuning of such oscillators is achieved typically by varying one or more of these circuit parameters so that the frequency of oscillation is changed for desired applications.
In the prior art, there are various wide-band electrically tunable solid-state microwave oscillators in which the oscillating frequency is controlled by variable reactance means. In general, these oscillators require the use of ferrite devices or varactors as the tuning element. Wide-band tuning has been achieved with a ferrite device, such as a YIG (yttrium-iron-garnet) sphere but the slowness of tuning due to the reaction in the magnetic circuit is a disadvantage. The added weight of the electromagnet to the structure and the power consumption of the unit are other problems related to these tunable oscillators using YIG spheres. Varactors are used for rapid tuning of microwave oscillators but the tuning range has been generally less than that of magnetic tuning. Increases in the tuning range in varactor tunable oscillators have been obtained, however, by sacrificing power output and oscillator performance efficiency.
Because high tuning rates are more desirable in present microwave systems and the development of electronic devices is toward more compact packages, the varactor tunable oscillator is more attractive than magnetically tuned devices. Many of the prior art devices have, therefore, used variations of varactor tunable circuits to increase the range of frequency tunability. Where the widest possible tuning range is required, a series circuit comprising a varactor connected in series with an active element and an impedance load has been used. As is well-known in the art, series tunable circuits are preferable for wide-band tuning since parallel tunable circuits result in inherently narrower tuning ranges.
U.S. Pat. No. 3,599,118, entitled "Varactor Tuned Negative Resistance Diode Microwave Oscillators," and issued on Aug. 10, 1971, discloses one type of prior art varactor tunable oscillator which includes a plurality of variable impedance devices connected in series with the transmission line center conductor to provide for tunability by modifying the effect of the reactance of the circuit to achieve the desired frequency modulation. U.S. Pat. No. 3,739,298, entitled "Broad Band Tunable Solid-State Microwave Oscillator," and issued on June 12, 1973, describes a varactor connected in series between first and second conductors of a transmission line wherein one or both conductors are adjustably mounted to effect adjustment of the characteristic impedance of the line and thus frequency tunability of the device. The tuning ranges for the devices described in these two patents as a percentage of the center frequency of tunable bandwidth are 662/3 percent and 50 percent, respectively. The generated frequencies of these oscillators, however, are within the range of 3 GHz to 11 GHz. One embodiment of the present invention is directed to an oscillator which is tunable in the Ka band which is over a range from 26.5 GHz to 40.0 GHz, which provides particulary for improved operation of a varactor tunable oscillator at higher microwave frequencies.
U.S. Pat. No. 3,735,286, entitled "Varactor Tuned Coaxial Cavity Negative Resistance Diode Oscillator," and issued on May 22, 1973, discloses an oscillator in which one varactor is connected in series with the active element and another varactor is connected in shunt with the active element to provide for wide-band tunability of the oscillator. In Ka band, this varactor tunable device utilizing only the shunt varactor is stated to have a tuning range of approximately 6 percent viz., 2 GHz, of the electronic tunable bandwidth. The present invention is an improvement over this type of varactor tunable oscillator since it is capable, as will be described, of achieving a tuning range greater than 40 percent of the center frequency in Ka band for a particular preferred embodiment.
Another problem encountered by many of the prior art varactor tunable oscillators in attempting to achieve wide-band tuning in high microwave frequency bands is the degradation of performance due to the high resistance of the varactors used in the oscillator circuit. In a series circuit, wide-band tuning is more desirably achieved by varactors which have large capacitance ratios, that is, the ratio of the maximum variable capacitance to the minimum. The so-called hyperabrupt varactors possess large capacitance ratios lending themselves to wide-band tuning applications but these varactors also have a relatively high resistance value. Where the resistance of the varactor, which is in series with the active element, exceeds the absolute value of the negative resistance of the active element, the oscillator performance in terms of output power and efficiency will be degraded. In particular, IMPATT diodes, which are used in oscillators advantageously for their high power and other improved operating characteristics, have relatively low associated negative resistance values. When hyperabrupt type high resistance varactors are used in combination with such low negative resistance IMPATT diodes, the oscillator performance will degrade significantly and the tunability will be ineffective.
The present invention is an improvement over known varactor tunable oscillators in that it provides for wideband tunability by utilizing in combination with two inductors a varactor having an abrupt junction which has a capacitance ratio less than the hyperabrupt type varactor, and also provides for stability of oscillator performance due to the lower resistance values associated with the abrupt type varactor.